Answer:
100
Step-by-step explanation:
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A) The constant of proportionality in this proportional relationship is 
B) The equation to represent this proportional relationship is y = 0.2x
<h3><u>Solution:</u></h3>
Given that,
The amount Naomi pays each month for international text messages is proportional to the number of international texts she sends that month
Therefore,
This is a direct variation proportion

Let "y" be the amount that Naomi pays each month
Let "x" be the number of international texts she sends that month
Therefore,

y = kx -------- eqn 1
Where, "k" is the constant of proportionality
Thus the constant of proportionality in this proportional relationship is:

<em><u>Last month, she paid $3.20 for 16 international texts</u></em>
Therefore,
y = 3.20
x = 16
Thus from eqn 1,

Substitute k = 0.2 in eqn 1
y = 0.2x
The equation would then be y = 0.2x